Optical beamsplitter

ABSTRACT

An optical beamsplitter comprises a right angle prism having a hypotenuse and two legs wherein the hypotenuse includes an optically selective coating deposited thereon. An optical element is positioned adjacent the hypotenuse such that the hypotenuse and the optical element form a beamsplitting interface. Various optical elements are utilized to achieve the desired spatial and angular relationships between incoming and outgoing light beams. Several embodiments are disclosed wherein the optical element can comprise a flat plate for providing spatial offset, a wedged plate for producing spatial and angular offset, a lens for adding optical power, or a faceted thin plate.

CROSS REFERENCE OF RELATED APPLICATIONS

This is a division of application Ser. No. 07/705,780, filed May 28,1991, now abandoned.

FIELD OF THE INVENTION

The invention relates to optical beam splitters/combiners.

BACKGROUND OF THE INVENTION

In many optical systems, it is often useful to separate and/or combinelight beams having different optical characteristics, e.g., polarizationstates, wavelengths, etc. For example, in optical data storage systems,the optical qualities of the storage medium are altered in a manner suchthat changes in the reflective or transmissive properties of the mediumare representative of the information recorded thereon. This informationis commonly transmitted to and retrieved from the optical medium usinglight beams produced by a laser light source.

The information recorded on the disc is retrieved from the disc bydirecting a laser beam onto the disc. The reflected laser beam is thendirected onto the detecting surface of a photodiode or other lightdetector system which transforms the reflected or transmitted laser beamsignal into an electrical signal. In this manner, the data stored on thedisc is transferred from the disc to the laser beam and converted intoan electrical signal which carries the same information recorded on thedisc. This electrical signal is further processed, and ultimatelyresults in retrieval of the computer data, audio sound, video images,etc., represented by the information recorded on the disc.

Separating and combining optical beams is useful in several ways in suchoptical disc systems. For example, in many optical disc memory systems,it is desirable to use a single laser source to produce both the readand write beams, thus resulting in a smaller and more compact systemdesign. However, when using a single laser source, it then becomesnecessary to be able to separate and distinguish the two beams. Inaddition, read only systems are often utilized in connection withoptical discs which require separation of the incident and reflectedread beams. Furthermore, it is necessary to separate the twopolarization states of the reflected read beam to detect the data storedon the disc.

The most common technique used to achieve these separations utilizescombinations of cube beamsplitters and right angle prisms. The cubebeamsplitter has a center interface selected such that it is sensitiveto the desired parameter of interest, i.e., polarization, wavelength,etc. The beamsplitter is then typically combined with one or more rightangle prisms attached to selected faces of the cube used to create thedesired separation/combination geometry. This type of design is oftentoo bulky and/or too heavy for use in many miniaturized optical heads.In addition, this separation technique requires that the beam bedetected by separate, or widely separated, detectors, thereby increasingthe volume requirements of the optical system.

A second technique frequently used to separate light beams involvesconnecting a right angle prism to a large cube. The angles of the largercube are selected such that the transmitted beam circulates around thecube and exits the cube at an angle which is offset with respect to thereflected beam. This design, too, is typically very heavy and requirestoo much space for incorporation into miniature optical heads, therebymaking it inefficient in many applications.

SUMMARY OF THE INVENTION

The present invention provides a small, compact beamsplitter which canbe used in a variety of applications. The optical beamsplitter of thepresent invention comprises a right angle prism having a hypotenuse andtwo legs wherein the hypotenuse includes an optically selective coatingdeposited thereon. An optical element is positioned adjacent thehypotenuse such that the hypotenuse and the optical element form abeamsplitting interface. Incoming light is transmitted so as to beincident upon the interface and is separated or combined in accordancewith the optical characteristics of the coating. In some preferredembodiments, the coating comprises a polarization sensitive coating suchthat the incoming light is separated or combined in accordance with thepolarization components of the light incident upon the interface. Theoptical element adjacent the hypotenuse is selected so as to produce thedesired spatial and/or angular relationships between theseparated/combined beams. In some embodiments, these optical elementsinclude flat plates used to provide spatial separation between beams,wedged plates which produce angular separation between beams, and lenseswhich add optical power to the beams.

The present invention provides an optical beamsplitter comprising afirst right angle prism including a first leg having a first length anda second leg having a second length wherein the first and second legsare joined by a hypotenuse. A thin optical element is positionedadjacent the hypotenuse. The optical element has a front surface and arear surface defining a maximum thickness which is less than the firstand second lengths of the first and second legs of the prism. Anoptically selective coating is positioned intermediate the hypotenuseand the front surface of the optical element to form a firstbeamsplitting interface. The optical element may have a first thicknessat a first location and a second thickness at a second location whereinthe first thickness is not equal to the second thickness. The rearsurface of the optical element may be flat. The rear surface of theoptical element may also be multi-faceted. The optically selectivecoating may comprise a polarization sensitive coating. The opticallyselective coating may also comprise a wavelength sensitive coating. Theoptical beamsplitter may further include a second right angle prismhaving a hypotenuse and two legs wherein the hypotenuse of the secondprism is positioned adjacent the rear surface of the thin opticalelement. The second right angle prism may have a partially reflectivecoating deposited on the hypotenuse to form a second beamsplittinginterface. The front and rear surfaces of the thin optical element maybe parallel. Further, the rear surface of the thin optical element mayhave a coating deposited thereon.

In one aspect of the invention, an optical beamsplitter is disclosedcomprising a first right angle prism having first and second legs joinedby a hypotenuse wherein the first leg has a first length and the secondleg has a second length wherein the hypotenuse includes an opticallyselective coating deposited thereon. The beamsplitter further comprisesa wedged plate having a front and a rear surface wherein the front andrear surfaces are non parallel. The front surface of the plate ispositioned adjacent the hypotenuse. The plate has a maximum thicknesswhich is less than the first and second lengths. The optically selectivecoating may comprise a polarization sensitive coating. The opticallyselective coating may also comprise a wavelength sensitive coating. Therear surface of the wedged plate may have a reflective coating depositedthereon.

In another aspect of the invention, an optical beamsplitter is disclosedwhich comprises a first right angle prism having first and second legsjoined by a hypotenuse wherein the first leg has a first length and thesecond leg has a second length and the hypotenuse includes an opticallyselective coating deposited thereon. The beamsplitter further comprisesa lens having a front surface and a rear surface wherein the frontsurface of the lens is positioned adjacent the hypotenuse. The lens hasa maximum thickness which is less than the first and second lengths. Theoptically selective coating may comprise a polarization sensitivecoating. The optically selective coating may also comprise a wavelengthsensitive coating. The rear surface of the lens may have a reflectivecoating deposited thereon.

In yet another aspect of the invention, an optical beamsplitter isdisclosed which comprises a first plate, a second plate positionedadjacent the first plate, and an optically selective coating positionedintermediate the first and second plates to form a beam splittinginterface. The optically selective coating may comprise a polarizationsensitive coating. The second plate may include a front surface and arear surface wherein the front surface is positioned adjacent the firstplate and the rear surface has a reflective coating deposited thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an optical disc memory systemincorporating a half-cube beamsplitter comprising a right-angle prism, awedged plate, and a quarter wave plate in accordance with the presentinvention;

FIG. 2 is a schematic illustration of a half-cube beamsplittercomprising a right-angle prism and a flat plate used to transmit lightbeams of two different wavelengths emitted from a dual wavelengthsource;

FIG. 3 is a schematic view of a half-cube beamsplitter comprising aright-angle prism and a flat plate which produces two outgoing beamsfocussed at separate focal points;

FIG. 4 is a schematic view of an alternative embodiment of a half-cubebeamsplitter comprising a right-angle prism and a wedged plate whichproduces two outgoing beams focussed at separate focal points;

FIG. 5 is a schematic representation of a half-cube beamsplittercomprising a right-angle prism and a flat plate which separates anincoming beam into two parallel offset outgoing beams;

FIG. 6 is a schematic illustration of a half-cube beamsplittercomprising a right-angle prism and wedged plate which separates anincoming beam into two angularly offset outgoing beams;

FIG. 7 is a schematic view of a half-cube beamsplitter comprising a lensand a right-angle prism which provides an outgoing beam having a pathparallel to an incoming beam and adds optical power to the outgoingbeam;

FIG. 8 is a schematic representation of a beamsplitter comprising tworight-angle prisms which separates an incoming beam into two outgoingbeams;

FIG. 9 is a schematic illustration of a beamsplitter comprising aright-angle prism and a flat plate which produces an outgoing beamangularly offset from an incoming beam;

FIG. 10 is a schematic representation of a beamsplitter comprising twoflat plates which separates an incoming beam into two outgoing beamswith parallel offset;

FIG. 11 is a schematic representation of a beamsplitter comprising aright-angle prism and faceted thin plate.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a half-cube beamsplitter 20 in accordance with thepresent invention as incorporated in an optical data storage system. Thesystem includes a source 22 and a detector 24 constructed as a singleunit or package 26. A collimator lens 30 is positioned intermediate thesource/detector package 26 and the beamsplitter 20. The beamsplitter 20comprises a right angle prism 32 having three sides, which as viewedfrom the top in FIG. 1, appear as a hypotenuse 34 and two legs 36, 38 Ofa right-angle triangle, wherein the first leg 36 faces thesource/detector package 26 and the second leg 38 faces an informationmedium, such as an optical disc 40. The beamsplitter 20 further includesa wedged plate 42 having a front surface 43 and a rear surface 45. Thelegs of the prism are preferably approximately 6 mm, and the maximumthickness of the wedged plate 42 is less than the length of the legs ofthe prism 32 and is preferably in the range of 0.5 to 2 mm. The wedgedplate 42 is located adjacent to the hypotenuse 34 of the right angleprism 32. A quarter wave plate 44 is adjacent to the second leg 38 ofthe prism 32 and a convex objective lens 46 is positioned intermediatethe quarter wave plate 44 and optical disc 40.

It should be understood that although specific dimensions have beengiven for a preferred embodiment, the proportions of the prism 32 andplate 42 could be scaled for implementation in numerous other opticalsystems in a variety of applications. For example, in an optical systemwith larger physical dimensions, the dimensions of the prism and platecould be made larger while still maintaining the same proportionalrelationship, and conversely, in an optical system with smaller physicaldimensions, the dimensions of the prism and the plate could be madesmaller while still maintaining the same proportional relationship.

The source 22 typically comprises a semiconductor laser which emitsdiverging light 49 through the collimating lens 30. The collimating lens30 renders the incoming light rays 49 substantially parallel andtransmits the collimated light 50 toward the beamsplitter 20. Thehypotenuse 34 of the prism 32 is coated with a polarization sensitivecoating at a beam splitting interface 52 formed between the hypotenusesurface 34 of the prism 32 and the front surface 43 of the wedged plate42. In this configuration, collimated laser light, incident upon thebeam splitting interface, is preferably linearly polarized such that thelight will be reflected by the beamsplitter 20 toward the quarter waveplate 44. The beamsplitter 20 is preferably oriented such that theincoming light beam 50 is incident upon the interface 52 at a 45 degreeangle, and is thus orthogonally deflected toward the quarter wave plate44. Upon passing through the quarter wave plate 44, the incidentlinearly polarized light becomes circularly polarized. This collimatedcircularly polarized light 54 is then focused onto the surface of theoptical disc 40 by the objective lens 46.

Assuming the beam is properly focussed on the recorded surface of theoptical disc 40, a reflected beam 56 will be imaged back onto the samepath as the incident light 54. As is well known to those skilled in theart, the reflected beam 56 is modulated in accordance with theinformation recorded upon the surface of the disc 40. Upon reflectionfrom the disc 40, the modulated reflected beam 56 is transmitted backthrough the objective lens 46 and quarter wave plate 44. The quarterwave plate 44 renders the circularly polarized reflected beam 56linearly polarized, but phase shifted by 90 degrees, one quarter of awavelength, from the incident linearly polarized beam 50. As the nowlinearly polarized reflected beam 58 strikes the beam splittinginterface 52, it is transmitted through the interface 52 to the rearsurface 45 of the wedged plate 42. Because of the wedge angle betweenthe front and rear surfaces 43, 45, the polarized reflected beam 58strikes the rear surface 45 of the plate 42 at an angle greater than 45degrees and is thus reflected at an angle with respect to the incomingbeam 50 toward the lens 30. The lens 30 causes the reflected beam 58 toconverge at the detector 24. The angle between the front and rearsurfaces 43, 45 of the wedged plate 42 is selected so as to cause thereflected light beam 58 to converge on the detector 24. The detector 24is typically a photodetector which transforms the optical signal into anelectrical signal carrying the same information encoded on the disc 40and contained in the modulated reflected light beam 58. In this manner,the beamsplitter 20 of the present invention functions to provideseparate optical paths for the transmitted and reflected beams 50, 58such that the source 22 and detector 24 can be located in closeproximity to one another in the same package 26. For use in optical discmemory systems, this provides a significant reduction in total weightand space of the optical head which carries the source/detector package26 and beamsplitter 20.

FIG. 2 illustrates how a half-cube beamsplitter 120 can be utilized totransmit beams of two distinct wavelengths. The beamsplitter comprises aright-angle prism 132 having a hypotenuse 134 and two legs 136, 138. Aflat plate 142 is placed adjacent to the hypotenuse 134 of the prism132. The hypotenuse 134 forms a beamsplitting interface 152 and iscoated so as to be sensitive to a particular wavelength, for example, toreflect a particular wavelength and transmit all other wavelengths ofincoming light. A dual wavelength source 122 emits two diverging lightbeams 150, 160 with two distinct wavelengths. For example, in opticaldata system applications, this configuration could be used to combinethe transmission of read and write beams through a single optical head.The beams 150, 160 are transmitted through a collimator lens 130 havinga central optical axis 162. Upon exiting the collimator lens 130, eachof the beams 150,160 is collimated but at different angles with respectto the central axis 162 of the lens 130, i.e., the collimated light raysof beam 150 form an angle of φ₁ with respect to the optical axis 162while the collimated light rays of beam 160 form an angle φ₂ withrespect to the axis 162. The collimated, angled beams 150, 160 aredirected through the first leg 136 of the prism 132 and are incidentupon the beam splitting interface 152.

The beam splitting interface 152 is coated with a wavelength sensitivecoating which reflects a selected wavelength of incident light andtransmits light of all other wavelengths. Alternatively, the coating maybe designed to transmit light of a selected wavelength and reflect allother wavelengths. Upon striking the interface 152, the incident beam150 having the first, selected wavelength is reflected off the interface152 at an angle equal to the angle of incidence forming a first outgoingbeam 164, and the incident beam 160 having the second, non-selectedwavelength is transmitted through the interface 152 to a rear surface143 of the flat plate 142. The length of the legs 136, 138 is preferablyapproximately 6 mm and the thickness of the plate 142 is substantiallyless than the length of the legs 136, 138 of the prism 132 and ispreferably in the range of 0.5 to 2 mm. Although specific dimensionshave been given for a preferred embodiment, the proportions of the prism132 and plate 142 could be scaled for implementation in numerous otheroptical systems in a variety of applications. At the rear surface 143 ofthe plate 142, the beam 160 having the second wavelength is reflectedforming a second outgoing beam 166. This could be accomplished numerousways including coating the rear surface 143 of the plate 142 with ahighly reflective coating, or orienting the plate 142 such that totalinternal reflection occurs and the beam 160 is reflected at an angleequal to the angle of incidence.

After exiting the beamsplitter 120, the two beams 164, 166 remaincollimated but are angularly offset and displaced depending on theincoming angle of incidence and the thickness and wedge angle of theplate 142. For application in reading and writing data to an opticaldisc, the first beam 164 could be focussed with a converging lens to afirst point on the disc and the second beam 166 could be focussed to asecond point on the disc, one beam serving to read information from thedisc and the other beam serving to write information to the disc. Oneskilled in the art will further realize that this configuration could bereadily applied to any optical system where it is desired to provideseparate optical paths for two light beams emitted from a dualwavelength source.

FIG. 3 shows a beamsplitter configuration 220 in accordance with thepresent invention which splits an incoming light beam 250 into twooutgoing light beams 264, 266 focussed on two spatially separate focalpoints 268, 270. The beamsplitter 220 comprises a right angle prism 232having a hypotenuse 234 and two legs 234, 236, wherein a flat plate 242is attached to the hypotenuse 234. As with the embodiment illustrated inFIG. 2, the plate 242 has a thickness which is substantially less thanthe length of the legs 236, 238 of the prism 232. Preferably, the lengthof the legs 236, 238 is approximately 6 mm and the thickness of theplate 242 ranges from 0.5 mm to 2 mm. The hypotenuse 234 forms abeamsplitting interface 252 having an optically selective coatingdeposited thereon. A lens 230 positioned adjacent the first leg 236 ofthe prism 232. It should be understood that although specific dimensionshave been given for a preferred embodiment, the proportions of the prism232 and plate 242 could be scaled for implementation in numerous otheroptical systems in a variety of applications.

The incoming beam 250 enters the lens 230 collimated and exits the lens230 in converging rays. The converging rays then enter the beamsplitter220 through the first leg 236 and are incident upon the beamsplittinginterface 252. The interface 252 is coated with an optically selectivecoating, such as a polarization sensitive coating which splits theincoming beam 250 so that light having a first, selected polarizationcomponent is reflected at an angle equal to the angle of incidence toform the first outgoing beam 264, and light having a second,non-selected polarization component is transmitted. Alternatively, thecoating could be wavelength selective so as to reflect/transmit aselected wavelength of incoming light. The transmitted light is incidentupon the rear surface of the flat plate 242 and is reflected, formingthe second outgoing beam 266. Light reflection at the rear surface canbe accomplished in a variety of ways including application of a highreflectivity coating to the rear surface and selection of the plate 242to effect total internal reflection as described above. Both beams264,266 exit the beamsplitter 220 through the second leg 238 of theprism 232 in two separate sets of converging rays which can be focussedupon two distinct points 268, 270. For example, the two beams 264, 266could be focussed on two different portions of an optical disc to reador write data on the disc, wherein the distance between the focal pointsis determined by the thickness of the plate 242. In addition, the beams264, 266 could represent portions of the reflected read beam which arefocussed upon separate photodetectors for application in focus errordetection.

A similar arrangement is shown in FIG. 4, however, in thisconfiguration, a focussing lens 330 is positioned after the beamseparation occurs. This arrangement can also be applied to optical discsystems where it is desired to provide two separate read/write beams orto separate the polarization states of a return beam for use in servotechniques. In this configuration, a collimated beam of light 350 isincident upon a half-cube beam splitter 320. The beam splitter 320comprises a right-angle prism 332 having two legs 336, 338 and ahypotenuse 334, wherein a wedged plate 342 is attached to the hypotenuse334. Preferably, the length of the legs 336, 338 is approximately 6 mm.The maximum thickness of the wedged plate 342 is in the range of 0.5 to2 mm and is substantially less than the lengths of the legs 336, 338. Itshould be understood that although specific dimensions have been givenfor a preferred embodiment, the proportions of the prism 332 and plate342 could be scaled for implementation in numerous other optical systemsin a variety of applications. The hypotenuse 334 forms a beam splittinginterface 352 having an optically selective coating, such as apolarization or wavelength sensitive coating, deposited thereon. Uponentering the beam splitter 320 through the first leg 336, the incomingbeam 350 is split such that a first portion of the beam 350 having afirst, selected polarization state is reflected off the interface toform a first outgoing beam 364 while a second portion of the beam 350having a second, non-selected polarization state is transmitted throughthe interface 352. The incoming beam 350 is incident upon the beamsplitting interface 352 at a 45 degree angle and is orthogonallyreflected toward the second leg 336 of the prism 332. The transmittedportion of the collimated beam 350 is incident upon the rear surface ofthe wedged plate 342 and deflected off the plate 342 using a reflectivecoating, total internal reflection, or other known methods, and forms asecond outgoing beam 366. The wedged plate 342 causes the angle ofincidence of the transmitted portion of the beam 350 to vary withrespect to the angle of the wedge, thus causing the portion of the beam350 reflected off the rear surface of the plate 342 to be angularlyoffset from the portion of the beam 350 reflected off the interface 352.The two outgoing beams 364, 366 then exit the beam splitter 320 incollimated rays which are angularly offset and are transmitted throughthe focussing lens 330. After exiting the lens, the outgoing beams 364,366 are caused to converge on two distinct focal points 368, 370, which,in an optical storage system, may advantageously be two detectors forgenerating focus error signals, or two read or write locations on thesurface of an optical disc.

A half-cube beam splitter in accordance with the present invention isshown in several additional embodiments in FIGS. 5 through 9. In eachembodiment below, the beam splitter comprises a right angle prism havinga hypotenuse joined to first and second legs. The hypotenuse forms abeam splitting interface and typically includes a polarization sensitivecoating deposited thereon, however, as in the embodiments describedabove, the interface could have any optically selective coating thereonto separate beams in accordance with any desired optical property of theincident beam. For example, a wavelength selective coating may be usedto separate incoming beams in accordance with the wavelength of theincident light. As will be explained in more detail below, various thinoptical elements are attached to the hypotenuse to achieve the desiredspatial and angular relationships between the incoming and outgoingbeams. As in previous embodiments, the lengths of the legs of the prismis preferably approximately 6 mm. Each optical element described belowpreferably has a maximum thickness which is less than the lengths of thelegs of the prism and is preferably in the range of 0.5 to 2 mm. Itshould be understood that although specific dimensions have been givenfor preferred embodiments, the proportions of the prism and opticalelements could be scaled for implementation in numerous other opticalsystems in a variety of applications.

In FIG. 5, a half-cube beam splitter 420 in accordance with the presentinvention is used to separate an incoming beam 450 into first and secondoutgoing beams 464,466 having a parallel offset with respect to eachother. In the context of optical data storage systems, thisconfiguration could be advantageously used to separate a read beam and awrite beam which carry data to and transmit data from an optical disc.The beam splitter 420 comprises a right angle prism 432 having two legs436, 438 and a hypotenuse 434, and a flat plate 442 which is attached tothe hypotenuse 434. The hypotenuse 434 of the prism 432 forms a beamsplitting interface 452 and is coated with an optically selectivecoating, preferably a polarization sensitive coating. The incoming beam450 is transmitted through the first leg 436 of the prism 432 and isincident upon the polarization sensitive interface 452 at a 45 degreeangle such that the beam 450 is split into a first portion of theincoming beam 450 having a first, selected polarization state which isorthogonally reflected off the interface 452, forming the first outgoingbeam 464, and a second portion of the incoming beam 450 having a second,non-selected polarization state which is transmitted through theinterface 452.

The second portion of the incoming beam 450 which is transmitted throughthe interface 452 is incident upon the rear surface of the flat plate442 and is reflected toward the second leg 438 of the prism 432 to formthe second outgoing beam 466. The beam splitter 420 and plate 442 arepreferably oriented such that the beam strikes the rear surface at a 45degree angle and total internal reflection occurs, thus causing thetransmitted portion of the incoming beam 450 to be orthogonallyreflected. The two outgoing beams 464, 466 then exit the second leg 438of the prism 432 separated by polarization components. Furthermore, theexiting beams 464, 466 are spatially offset and travel in parallelplanes. In an optical disc data system, these beams could be focussedupon an optical disc in order to simultaneously read and write data,although, one skilled in the art will recognize this configuration couldbe used in a variety of applications where it is desired to separate anincoming light beam into two outgoing light beams.

Another embodiment of a beam splitter 520 of the present invention isshown in FIG. 6. This configuration produces beam separation with anangular offset. The beam splitter 520 comprises a right-angle prism 532having a hypotenuse 534 and two legs 536, 538, and a wedged plate 542attached to the hypotenuse 534. The hypotenuse 534 forms a beamsplitting interface 552 and is coated with a polarization sensitivecoating, or other type of optically sensitive coating, such as awavelength sensitive coating. In this embodiment, an incoming beam 550is transmitted through the first leg 536 of the prism 532 and isincident upon the polarization sensitive interface 552. As in previousembodiments, the incoming beam 550 is split such that a first portion ofthe incoming beam 550 having a first, selected polarization state isreflected off the interface 552, forming a first outgoing beam 564, anda second portion of the incoming beam 550 having a second, non-selectedpolarization state is transmitted through the interface 552. Theincoming light 550 is preferably incident upon the beam splittinginterface 552 at a 45 degree angle such that the first outgoing beam 564is orthogonally reflected toward the second leg 538 of the prism 532.The transmitted portion of the beam 550 is incident upon the rearsurface of the wedged plate 543 and is reflected, forming a secondoutgoing beam 566. Because of the varying thickness of the wedged plate542, the transmitted portion of the beam 550 is incident at an anglegreater than 45 degrees and, thus, is reflected at an angle greater than45 degrees. Thus, the portion of the beam reflected off the rear surfaceof the plate 542 is reflected at an angle with respect to the portion ofthe beam reflected off of the beam splitting interface 552. In thismanner, the outgoing beams 564, 566 exit the beam splitter 520 separatedby polarization states and angularly offset from each other.

FIG. 7 illustrates yet another embodiment of a half-cube beam splitter620. The beam splitter 620 comprises a right angle prism 632 having ahypotenuse 634 and two legs 636, 638, wherein a lens 642 is attached tothe hypotenuse 634. The hypotenuse 634 is coated with an opticallyselective coating, and preferably, with a polarization sensitivecoating. A beam of laser light 650, transmitted through the first leg636 of the prism 632 and incident upon the beam splitting interface 652,is preferably polarized such that the light will be reflected. The beamsplitter 620 is oriented such that the incoming light beam 650 isincident upon the interface 652 at a 45 degree angle, and is thusorthogonally deflected toward the second leg 638 of the prism 632. Foruse in optical disc memory systems, the exiting beam 650 could then bedirected toward an optical disc. Typically the beam 650 is reflectedfrom the surface of the disc along the path of the incident light andaltered in polarization by a quarter-wave plate such that the reflectedbeam 658 is phase-shifted by 90 degrees upon entering the beam splitter620. As the phase-shifted beam 658 strikes the beam splitting interface652, it is now transmitted through the interface 652 to the rear surfaceof the lens 642. The rear surface of the lens 642 is coated with areflective coating and the reflected beam 658 is deflected in adirection approximately parallel to the original path of the incidentlight, while further adding optical power to the reflected read beam 658via the lens 642.

A further embodiment of a beam splitter 720 in accordance with thepresent invention is illustrated in FIG. 9. The beam splitter 720comprises two right-angle prisms 732,732', each having a hypotenuse 734,734', and two legs 736, 736', and 738,738'. The prisms 732,732' arepositioned such that the hypotenuse 734 of the first prism 732 faces thehypotenuse 734' of the second prism 732' and are separated by a piece ofglass 735. The hypotenuse 734 of the first prism 732 forms a first beamsplitting interface 752 having a polarization sensitive coatingdeposited thereon. The hypotenuse 734' of the second prism 732' forms asecond beam splitting interface 752' which is coated with a partiallytransmissive/partially reflective coating. An incoming light beam 750strikes the first beam splitting interface 752 and is preferablypolarized and incident upon the interface 752 at a 45 degree angle suchthat it is orthogonally deflected toward the second leg 738 of the firstprism 732. In an optical disc memory system of the type shown in FIG. 1,this beam could be directed through a quarter wave plate toward thesurface of an optical disc.

The beam is reflected from the optical disc along the same path as theincident light beam 750 and enters the beam splitter 720 one-quarterwavelength, 90 degrees, out of phase from the original beam 750. Uponstriking the first beam splitting interface 752, the reflected beam 758is now transmitted through the first interface 752 to the second prism732' and is incident upon the second beam splitting interface 752'. Thesecond interface 752' allows a first portion of the reflected beam 758to be transmitted through the interface 752', forming a first outgoingbeam 770 which exits the beam splitter 720 through the first leg 736' ofthe second prism 732', and a second portion of the beam 758 to bereflected off of the interface 752'. The reflected beam 758 is incidentupon the second interface 752' at a 45 degree angle such that the secondportion of the reflected beam 758 is orthogonally deflected along a pathparallel to the incident light beam 750, thereby forming a secondoutgoing beam 772. This configuration can be utilized in optical discdata systems to separate and analyze a portion of the returning readbeam in many well-known focus error detection techniques.

Yet another embodiment of a half-cube beam splitter 820 is shown in FIG.8. As shown, the beam splitter 820 comprises a right-angle prism 832having two legs 836, 838, a hypotenuse 834, and a flat plate 842 whichis attached to the prism 832 at the hypotenuse 834. The hypotenuse 834forms a beam splitting interface 852 and has a polarization sensitivecoating deposited thereon. An incoming light beam 850 is transmittedthrough the first leg 836 of the prism 832 and incident upon theinterface 852 at a 45 degree angle. The beam 850 is preferably polarizedsuch that the light is orthogonally deflected toward the second leg 838of the prism 832. In the context of optical disc data systems, thedeflected beam 850 could be directed so as to read data recorded on thesurface of an optical disc. In such a system, the beam is reflected fromthe disc along the same path as the incident light beam 850 back towardthe beam splitter 820. The reflected beam 858 is phase shifted inpolarization such that, upon striking the interface 852, the reflectedbeam 858 is transmitted through to the rear surface of the plate 842. Atthe rear surface of the plate 842, the transmissive coating allows thereflected beam 858 to be transmitted through the beam splitter 820. Dueto the differences in index of refraction between the plate 842 and theair, the transmitted beam 858 exits the plate 842 at an angle. The beam858 could then be advantageously directed toward a photodetector foranalysis of the data read from the disc and/or used in well-known focuserror detection methods.

FIG. 10 illustrates a further embodiment of the invention. A beamsplitter 920 is shown comprising two flat plates 980,982 preferablyhaving a thickness in the range of 0.5 to 2 mm which are joined at abeam splitting interface 984. The interface 984 is coated with apolarization sensitive coating. An incoming light beam 990 istransmitted through the first plate 980 and is incident upon theinterface 984 such that the incoming beam 990 is split. A first portionof the beam 990 having a first, selected polarization component isreflected, forming a first outgoing beam 992, and a second portion ofthe beam 990 is transmitted through the interface 984 to the rearsurface of the second flat plate 982. The rear surface of the plate 982has a reflective coating deposited thereon such that the transmittedportion of the beam 990 is deflected, forming a second outgoing beam994. The plates 980, 982 are oriented such that both portions of thebeam 990 are reflected at approximately the same angle. Upon exiting thebeam splitter 920, the difference in index of refraction between theplates 980, 982 and the air causes the beams 992,994 to exit at anangle. Thus, the beam splitter serves to separate the incoming beam 990into two parallel outgoing beams 992, 994 of different polarizationstates. In an optical disc data system, these two beams 992, 994 couldbe used to simultaneously read data from and write data to an opticaldisc, although this configuration could be used in other optical systemswhere it is desired to separate an incoming beam into two outgoingbeams.

Yet another embodiment of the invention is shown in FIG. 11. A beamsplitter 1020 is illustrated which comprises a right angle prism 1032having a hypotenuse 1034 which joins first and second legs 1036, 1038. Afaceted thin plate 1042, such as a roof-top prism, comprising a frontsurface 1043 and a multi-faceted rear surface 1045 having a firstportion 1047, and a second portion 1049 which is angled with respect tothe first portion 1047, is attached to the hypotenuse 1034 of the prism1032. The hypotenuse 1034 is coated with a polarization sensitivecoating, or other type of optically selective coating to form a beamsplitting interface 1052 intermediate the hypotenuse 1034 and roof-topprism 1042. A diverging incoming beam 1050 is transmitted through thefirst leg 1036 of the right angle prism 1032 and is incident upon thebeam splitting interface 1052. The diverging beam 1050 is shown forillustrative purposes only. Other source beams, such as collimated beamswith auxiliary optics could also be used. The incident beam 1050 ispreferably polarized such that the beam 1050 will be reflected off ofthe interface 1052 and transmitted through the second leg 1038 of theprism 1032. For use in optical disc memory systems, the exiting beamcould then be directed toward an optical disc. Typically the beam 1050is reflected from the surface of the disc as a reflected beam 1060 alongthe path of the incident light beam 1050 and altered in polarizationsuch that the reflected beam 1060 is phase-shifted by 90 degrees uponentering the beam splitter 1020 through the second leg 1038 of the rightangle prism 1032. As the phase-shifted beam 1060 strikes the beamsplitting interface 1052, it is now transmitted through the interface1052 to the rear surface 1045 of the roof-top prism 1042.

A portion of the reflected beam 1060 will be incident upon the firstportion 1047 of the rear surface 1045 of the roof-top prism 1042, and aportion of the reflected beam 1060 will be incident upon the secondportion 1049 of the rear surface 1045. Because the first portion 1047 ofthe rear surface is angled with respect to the second portion 1049 ofthe rear surface, the reflected beam 1060 is deflected from the rearsurface 1045 as shown to form two beams 1062, 1064, which converge attwo distinct focal points, 1072, 1074, respectively, one above and onebelow the source of the diverging beam 1050. If the tilt between the twoangled portions 1047, 1049 of the roof top prism 1042 was rotated 90degrees about the normal to the hypotenuse, then the return beams 1062,1064 would separate into and out of the page.

Although the invention is illustrated with reference to specificembodiments, it will be apparent to those skilled in the art thatvarious modifications could be made without departing from the truespirit of the invention. For example, other optically selective coatingsmay be used at the beam splitting interface in addition to thepolarization and wavelength sensitive coatings illustrated. For example,a differential frustrated total reflection (DFTR) coating whosetransmissive/reflective properties vary as a function of the angle ofincidence of incoming light could also be used. In addition, althoughmany of the embodiments are illustrated as beam splitters, one skilledin the art will realize that the same configuration could be reversedand used alternatively to combine beams. Furthermore, it should berecognized that the disclosed embodiments could be used with eithercollimated or focussed light.

The invention may be embodied in other specific forms without departingfrom its spirit or essential characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is, therefore, indicated by theappended claims rather than by the foregoing description. All changeswhich come within the meaning and range of equivalency of the claims areto be embraced within their scope.

I claim:
 1. An optical beam splitting assembly for use in an opticalsystem, said assembly comprising:a light source producing an incidentlight beam; a reflective medium for receiving said incident light beamand reflecting a corresponding reflected light beam; a right angle prismincluding a first leg having a first length and a second leg having asecond length, said first and second legs being joined by a hypotenuse,said first length and said second length being approximately equal toeach other; a thin faceted optical element positioned adjacent saidhypotenuse, said optical element having a front surface and a rearsurface, said thin faceted optical element having a maximum thicknesswhich is less than said first and second lengths of said first andsecond legs of said prism, said rear surface formed by a first portionand a second portion, said first and second portions being non-planarrelative to each other; an optically selective coating positionedintermediate said hypotenuse and said front surface of said thin facetedoptical element to form a beam splitting interface; and a light detectorwhich receives said reflected light beam wherein said incident lightbeam passes from said light source through said first leg, reflectingoff said beam splitting interface to pass through said second leg asdirected toward said reflective medium thereby producing saidcorresponding reflected light beam, said corresponding reflected lightbeam returning through said second leg and through said opticallyselective coating and reflecting off said first portion and a secondportion of said rear surface thereby forming a first converging returnbeam and a second converging return beam each passing through said firstleg.
 2. The optical beam splitting assembly according to claim 1 whereinsaid first converging return beam terminates at a first focal point anda second converging return beam terminates at a second focal point, saidfirst and second focal points being located proximate said light source.3. The optical beam splitting assembly according to claim 1 wherein saidlight source and said light detector are arranged in a compact unit. 4.The optical beam splitting assembly according to claim 1 wherein saidoptically selective coating is wavelength sensitive.
 5. The optical beamsplitting assembly according to claim 1 wherein said optically selectivecoating is polarization sensitive.
 6. The optical beam splittingassembly according to claim 1 wherein said rear surface of said thinfaceted optical element has a reflective coating deposited thereon. 7.The optical beam splitting assembly according to claim 1 wherein saidmaximum thickness of said thin faceted optical element is within therange of 0.5 mm to 2 mm.
 8. The optical beam splitting assemblyaccording to claim 1 wherein said first length and said second length ofsaid right angle prism are approximately equal to 6 mm.